1. Field of the Invention
The present invention generally relates to a method for fabricating a buried capacitor structure, and more particularly to a method for fabricating a structure with a capacitor buried in a circuit board.
2. The Prior Arts
Buried passives are passive components disposed between layers of a multi-layer circuit board. The electronic components, such as capacitors or resistors are directly formed on an inner layer of the circuit board by etching or printing. Then, at least one outer layer of the circuit board is laminated onto the inner circuit board to bury the electronic component inside the multi-layer circuit board. The buried passives are adapted to replace those discrete passives soldered to the circuit board, so as to free up space on the circuit board to pack more circuitry and active components.
Buried resistor technologies are first proposed by Ohmega Technologies, Inc., a manufacturer of OHMEGA-PLY® resistor-conductor material. The buried resistor is a thin film of a phosphorous-nickel alloy serving as a resistive element plated onto a matt side of a copper foil of an inner layer. Then, they are compressed to configure a thin core, and later processed by photo-resist processing twice and etching processing thrice, so as to configure a desired thin film resistor at a specific position. Such a thin film resistor is disposed between the layers, and thus called buried resistor.
Generally, a conventional capacitor structure includes a parallel plate capacitor, which has a source electrode and a ground electrode divided by a dielectric layer. Various approaches have been used to increase capacitance of the capacitor. Because capacitance of the capacitor is proportional to permittivity, one of the approaches is to provide the dielectric layer with a higher permittivity. Furthermore, capacitance of the capacitor increases with area and decreases with separation, and therefore another approach is to increase the area of the two electrodes and to decrease the distance between the two electrodes.
A conventional method for fabricating an embedded capacitor structure is illustrated in FIGS. 1 through 5. Referring to FIG. 1, a thin copper layer 4 is formed on a substrate 2, and then a patterned photo-resist layer 6 is formed on the thin copper layer 4 to expose a portion of the thin copper layer 4. Referring to FIG. 2, a comb-shaped electrode plate 10a is electroplated on the exposed portion of the thin copper layer 4, which is not covered by the photo-resist layer 6. Referring to FIG. 3, the comb-shaped electrode plate 10a includes a positive electrode 16 and a negative electrode 18. The positive electrode 16 includes a positive electrode end 12 and a plurality of positive comb branches 17, and the negative electrode 18 includes a negative electrode end 14 and a plurality of negative comb branches 19. The positive comb branches 17 and the negative comb branches 19 face each other and interleave without touching. The interleaving positive comb branches 17 and the negative comb branches 19 are parallel to each other, and keep a predetermined separation distance therebetween. Referring to FIG. 4, the photo-resist layer 6 is removed and a capacitor paste 15 is filled in a clearance between the positive electrode 16 and the negative electrode 18 to form a planar comb-type capacitor 10. Referring to FIG. 5, the capacitor 10, the thin copper layer 4 and the substrate 2 are laminated with a dielectric film 20 having a first metal layer 30. The capacitor 10 is embedded into a first surface of the dielectric film 20 and the first metal layer 30 is disposed on a second surface of the dielectric film 20. Then, the thin copper layer 4 and the substrate 2 are removed, thereby forming an embedded planar comb-type capacitor structure as shown in FIG. 5.
In the above-described configuration, parts of the positive electrode 16 and the negative electrode 18 are exposed on the first surface of the dielectric film 20, and thus electrical property of the capacitor 10 is affected by environmental factors. For example, high humidity can reduce breakdown voltage of the capacitor, and even damage the capacitor.
Hence, it is necessary to provide a method for fabricating a buried capacitor structure whose capacitor is insulated from environment and is not affected by environmental factors, and in the meanwhile has a smaller distance between the positive electrode and the negative electrode.